PERISELECTIVITY IN THE REACTIONS OF CYCLOPENTADIENONES WITH 8-ARYL-8-AZAHEPTAFULVENES

8-Aryl-8-azaheptafulvenes 2 reacted smoothly with 2,5-dimethyl-3,4-diphenylcyclopentadienone (1a) to give exo [8+4] adducts, i.e. 4, as the dominant adducts along with minor amounts of endo [4+2] adducts, i.e. 7 and 8, and trace amounts of [8+2] adducts, i.e. 6. Passing from 2,5-dimethyl to 2,5-dimethoxycarbonyl-3,4-diphenylcyclopentadienone (1c) brought about an increase in reaction rate without any relevant change in selectivity. Structures of the exo [6+4] adducts 4 rest firmly on spectroscopic data and chemical behavior. Upon heating the [6+4] adducts 4 underwent a 13,31 aza-Cope rearrangement followed by [1,5-H] sigmatropic shifts to afford the [8+2] adducts 6. Mechanisms of formation of all the adducts are discussed We conclude that only concerted cycloadditions are at work in the reactions of azaheptafulvenes with cyclopentadienones